Did you get a thorough physical examination from a qualified doctor before you began training?

Assuming that you're normal and healthy, dizziness is often caused by a lack of oxygen to the brain. (It can also be caused by inner ear problems, which is a medical condition.) Therefore, make sure that you are breathing when you roll and not holding your breath. If you hold your breath, of course after two or three rolls you are going to feel dizzy. Anyone would.

Breath in, and then as you roll, breath out. Relax. Stand up. Make sure that you have good balance and posture. Look straight ahead. Look at some fixed object on the wall. Breath, relax, and repeat the process.

Don't go too fast. Don't feel that you have to keep up with the class. If the instructor pushes you, tell him/her your situation. If they aren't being a macho hardass, they should understand.

As time goes on, you're dizziness should subside -- Then you can try going a little faster.

I also get the dizzy feeling if I haven't practiced rolling for awhile. I think it's natural. After a few sessions, I feel better.